1. Field of the Invention
The present invention relates to thermal printing and, more particularly, to techniques for improving thermal printer output by compensating for the effects of thermal history on thermal print heads.
2. Related Art
Thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment or dye from a donor sheet to the output medium or by activating a color-forming chemistry in the output medium. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the receiver. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power to the print head element within a particular time interval or by providing power to the print head element for a longer time interval.
In conventional thermal printers, the time during which a digital image is printed is divided into fixed time intervals referred to herein as “print head cycles.” Typically, a single row of pixels (or portions thereof) in the digital image is printed during a single print head cycle. Each print head element is typically responsible for printing pixels (or sub-pixels) in a particular column of the digital image. During each print head cycle, an amount of energy is delivered to each print head element that is calculated to raise the temperature of the print head element to a level that will cause the print head element to produce output having the desired density. Varying amounts of energy may be provided to different print head elements based on the varying desired densities to be produced by the print head elements.
One problem with conventional thermal printers results from the fact that their print head elements retain heat after the conclusion of each print head cycle. This retention of heat can be problematic because, in some thermal printers, the amount of energy that is delivered to a particular print head element during a particular print head cycle is typically calculated based on an assumption that the print head element's temperature at the beginning of the print head cycle is a known fixed temperature. Since, in reality, the temperature of the print head element at the beginning of a print head cycle depends on (among other things) the amount of energy delivered to the print head element during previous print head cycles, the actual temperature achieved by the print head element during a print head cycle may differ from the desired temperature, thereby resulting in a higher or lower output density than is desired. Further complications are similarly caused by the fact that the current temperature of a particular print head element is influenced not only by its own previous temperatures—referred to herein as its “thermal history”—but by the ambient (room) temperature and the thermal histories of other print head elements in the print head.
As may be inferred from the discussion above, in some conventional thermal printers, the average temperature of each particular thermal print head element tends to gradually rise during the printing of a digital image due to retention of heat by the print head element and the over-provision of energy to the print head element in light of such heat retention. This gradual temperature increase results in a corresponding gradual increase in density of the output produced by the print head element, which is perceived as increased darkness in the printed image. This phenomenon is referred to herein as “density drift.”
Furthermore, conventional thermal printers typically have difficulty accurately reproducing sharp density gradients between adjacent pixels both across the print head and in the direction of printing. For example, if a print head element is to print a black pixel following a white pixel, the ideally sharp edge between the two pixels will typically be blurred when printed. This problem results from the amount of time that is required to raise the temperature of the print head element to print the black pixel after printing the white pixel. More generally, this characteristic of conventional thermal printers results in less than ideal sharpness when printing images having regions of high density gradient.
The above-referenced patent applications disclose a model of a thermal print head that predicts the thermal response of thermal print head elements to the provision of energy to the print head elements over time. The amount of energy to provide to each of the print head elements during a print head cycle in order to produce a spot having the desired density is calculated based on: (1) the desired density to be produced by the print head element during the print head cycle, (2) the predicted temperature of the print head element at the beginning of the print head cycle, (3) the ambient printer temperature at the beginning of the print head cycle, and (4) the ambient relative humidity.
The techniques disclosed therein assume that printing is performed in equal time steps, and therefore calculate the input energy in equal time steps, each corresponding to the time taken to print a single pixel on the thermal medium. In particular, the disclosed techniques implement a thermal model for the thermal print head. The thermal model is composed of multiple layers, each having a different spatial and temporal resolution. The resolutions for the layers are chosen for a combination of accuracy and computational efficiency.
Furthermore, the techniques disclosed in the above-referenced patent applications implement a media model that computes the energy needed to print a desired optical density on the medium, given the current temperature profile of the print element. The media model is expressed in terms of two functions of the desired density, G(d) and S(d). G(d) corresponds to the inverse gamma function at a specified reference temperature, and S(d) is the sensitivity of the inverse gamma function to temperature at a fixed density.
The assumption that all print intervals are of equal duration may not be valid under all circumstances. For example, in the system disclosed in the above-referenced patent application entitled “Thermal Imaging System,” the print head is capable of writing two colors in a single pass on a single print medium. Each print line time is divided into two parts. It is possible to write one color in one part of the line time and another color in another part of the line time. The time division between the two colors, however, may not be equal. For example, if printing yellow and magenta, the yellow may be printed during a smaller fraction of the line time interval than magenta. An attempt to apply the thermal history control techniques disclosed above to such a print mechanism may, therefore, produce suboptimal results, because the assumption of equally-sized print intervals would be violated.
What is needed, therefore, are improved techniques for controlling the temperature of print head elements in a thermal printer having unequally-sized print intervals to more accurately render digital images.